We have recently described the infrared absorptions of the "hidden" alpha-104 and B-112 cysteinyl SH groups which are in the alpha 1 beta 1 interface of human hemoglobin, and have shown that these absorptions are sensitive to the state of hemoglobin ligation. Hemoglobin denaturation is a central feature of hemolytic diseases like thalassemia, unstable hemoglobinopathies and hexose monophosphate shunt deficiencies. Hemoglobin denaturation is a complex process with multiple molecular pathways which probably involves separation of the alpha and beta chain along the alpha 1-beta 1 interface in most cases. The alpha-104 and beta-112 SH groups are therefore excellent probes of molecular changes characteristic of denaturation as well as of conformational changes associated with normal ligand binding. The SH groups are also important in the aggregation and precipitation of denatured hemoglobin and possibly in the interaction of Heinz bodies. We propose to study the process of Hb denaturation and interaction with red cell membranes in vitro using multiple spectral techniques, with special emphasis on SH groups, and apply this information to in vivo studies. We also will study the molecular mechanisms of cooperative ligand binding by hemoglobin using various synthetic hemoglobins, hemoglobin variants and hemoglobin derivatives.